With the advent of solid state or electronic trip units for application in industrial circuit breakers to respond to overcurrent conditions ranging from overload to short circuit, it is now common to add the capability of responding to a ground fault condition. To sense overcurrent, a current transformer is coupled with each phase of a distribution circuit such as to develop a secondary output signal voltage proportional to the current flowing in that phase. These signal voltages are then processed to determine if the current in any phase is of overcurrent proportions. When an overcurrent condition is found to exist, a trip function is initiated with or without delay, depending on the magnitude of the sensed overcurrent.
To sense a ground fault condition, the total current flowing in the distribution circuit from the source to the load is compared with the total current returning from the load to the source. A ground fault exists if an imbalance or inequality in these currents is found, since some of the current leaving the source is then flowing through the ground fault and returning to the source via an extraneous ground path. One way of detecting this imbalance is to link the distribution circuit conductors with a differential current transformer core and look for a voltage induced in a secondary winding by residual flux flowing in the core as occasioned by an imbalance in the currents flowing to and from the load. Such a differential transformer must necessarily be large and relatively expensive in order to link with the plural line conductors of a polyphase distribution circuit, as well as the neutral conductor if present. Moreover, the secondary output must be handled by a separate input section, apart from the input section handling the secondary outputs from the phase overcurrent sensing transformers.
An alternative approach to ground fault sensing is to interconnect the secondary windings of the phase current transformers and the neutral current transformer, if a neutral conductor is present, in a summing circuit. If the summation of these secondary currents is zero, there is no ground fault condition present. However, if the summation of these secondary currents is not zero, then a ground fault condition is indicated. Prior executions of this approach have typically resulted in rather complex input section including multiple full wave rectifying networks and various circuit provisions for separating overcurrent information from ground fault information as manifested by the current transformer secondary outputs.
It is accordingly an object of the present invention to provide an improved input section for interfacing a plurality of current sensors with overcurrent trip and ground fault trip circuit networks included in a circuit breaker electronic trip unit.
An additional object is to provide a circuit breaker trip unit having an input section of improved construction for developing a ground fault signal proportional to the magnitude of ground leakage current.
Still another object of the invention is to provide a trip unit input section of the above-character for handling both the ground fault and overcurrent information contents in the outputs of the current sensors.
A further object is to provide a circuit breaker trip unit input section of the above-character which is efficient in design and reliable in operation.
Other objects of the invention will be part be obvious and in part appear hereinafter.